The present invention relates to a method for manufacturing a shadow mask for color CRT, and in particular to a method for manufacturing a thin-plate shadow mask of 20-80 .mu.m in thickness and to a shadow mask manufactured by said method, more specifically, an effective method for manufacturing a thin-plate type aperture grill in form of slits.
With the recent increase of large-size CRT display units for color television and the like, there are now strong demands on large-size shadow masks. To produce lightweight shadow masks, thin plate of 20-80 .mu.m in thickness is now widely used.
To manufacture a shadow mask of conventional type using a base material of 130-150 .mu.m in thickness, an electrode base material 1 is cleaned up as shown in FIGS. 3A-3H, and resist 2 is coated on both surfaces of the base material 1 (FIG. 3B). Using a glass mask 3, the resist 2 on both surfaces is exposed to light (FIG. 3C). After developing the resist 2, patterning is performed, and an etching film is produced through baking (FIG. 3D). Then, etching is performed from both surfaces of the base material 1. After forming an opening (FIG. 3E), the resist 2 is removed (FIG. 3F). This is called one-step etching method. If this method is applied to thin plate material of 20-80 .mu.m, surface flatness cannot be maintained because of spraying pressure during etching, and shape and dimensional accuracy of the etching hole thus completed are sometimes not satisfactory. There is another method, by which an electrode base material 1 is cleaned up (FIG. 4A), and resist 2 is coated on both surfaces of the base materials 1 as shown in FIGS. 4A-4H. Using a glass mask 3, the resist 2 on both surfaces is exposed to light (FIG. 4C). After developing the resist 2, patterning is performed, and an etching film is prepared through baking (FIG. 4D). Then, etching is performed only on one surface of the base material 1 to prepare hole (FIG. 4E). An anti-etching packing material 4 is filled into this hole (FIG. 4F), and etching is performed again from the side opposite to the hole (FIG. 4G), and the resist 2 is removed (FIG. 4H). This is called two-step etching method. When this method is applied to thin plate base material of 20-80 .mu.m, it is not possible to obtain cross-sectional shape as desired. The latter is disclosed in the Japanese Laid-Open Patent Publication No. 61-130492 and others.
When the thickness of the base material is as thin as 20-80 .mu.m as described above, the plate thickness is relatively too thin to form an etching hole of 60-250 .mu.m in width, and the desired cross-sectional shape cannot be prepared by the conventional two-step etching method as shown in FIGS. 4A-4H. (Etching hole penetrates the material before the desired cross-sectional shape is obtained.) By the one-step etching method shown in FIGS. 3A-3F, the product of good quality in terms of shape, linearity and dimensional accuracy of etching hole cannot be obtained because the base material is too thin to have sufficient strength and surface flatness cannot be maintained during etching.